For internal combustion engines, it is known not to vent fuel vapors to the ambient. These fuel vapors are formed in dependence upon specific parameters such as fuel temperature, fuel quantity, vapor pressure, air pressure, scavenging quantity and the like. Instead, it is preferable to pass the fuel vapors to the engine via an intermediate container filled with active charcoal. The active charcoal container receives the fuel vapors formed in the tank such as in a stationary vehicle. The active charcoal container is then usually connected via a line with the intake region of the engine and therefore passes fuel to the engine in addition to the fuel metered to the engine by the fuel metering system. This system determines the particular quantity of fuel required for the operation of the engine while considering specific operating characteristic quantities.
In this connection, it is also known to prevent an increase of the exhaust emissions or to hold the latter to low values in that the tank venting (TE) is only permitted for specific operating conditions of the engine. With respect to the foregoing, reference may be made to the publication of Robert Bosch GmbH entitled "Motronic - Technische Beschreibung" C5/1, Aug. 1981 and German published patent application DE-OS No. 28 29 958. The above-mentioned increase of the exhaust emissions is brought about by such an additional fuel-air mixture quantity caused by the venting of the tank.
The intermediate storage container containing the active charcoal filter can store fuel vapors up to a predetermined maximum quantity with a scavenging or regeneration of the filter occurring during operation of the engine by means o the under pressure developed by the engine in the air intake region. An additional fuel/air mixture which is accounted for by the tank venting therefore results also if the regeneration of the intermediate storage is permitted only for specific operating conditions. This fuel/air mixture is not measured or cannot be measured with a reasonable effort and falsifies the fuel metering signal which is normally very exactly determined with a high computation effort. This fuel metering signal can be an injection control command t.sub.i for a fuel injection apparatus or a positioning current associated with a system for continuous fuel injection. The foregoing causes the fuel/air mixture to falsify the fuel quantity supplied to the engine.
This means that for specific angles of the throttle flap, the lambda value can be very substantially influenced by the fuel flows from the tank venting. The tank venting therefore creates problems also if the influence of this disturbing quantity is referred to the intake pipe pressure developed by the engine by means of pneumatic positioning members or if one completely excludes the application of the tank venting mixture by means of an electronic control for especially sensitive operating conditions such as idle. The tank venting operation becomes especially questionable if the fuel metering system is a so-called learning system. The purpose of such a learning, adaptive injection system is not to control out relatively constant disturbing quantities (idle carbon monoxide, elevation errors, leakage air errors and the like) by means of conventionally available lambda controls; instead, these disturbing quantities are to be correctly precontrolled immediately with the aid of learned correction values. The basis for such a precontrol comprises that the average long-term deviation of the lambda integrator values from the neutral value .lambda.=1 is recognized and precontrolled quantities are adaptively so changed that a compensation of the disturbing quantities is possible. The long-term deviation is caused by specific disturbing quantities.
If the occurrence of an additional disturbance is traced to the undefined mixture of a tank venting apparatus which vents into the intake path of the engine, then the learning functions of the adaptive lambda precontrol must be turned off so that the precontrol quantities which have already been adapted and which are valid for normal operation without tank venting cannot be again falsified.
In this connection, there are two requirements which must be fulfilled. The adaptation (the learning of the drifts) must be repeatedly updated which in most instances can be done by means of adaptation with global (multiplicative) or structural (additive) operating factors. In special cases, adaptive learning characteristic fields are superposed on basic characteristic fields or the disturbing quantities (leakage air, elevation errors) must be learned out, for example, by systems which continuously meter or inject fuel. These disturbing quantities become manifest as offset or slope errors at the initial line .lambda.=1.
The above-mentioned injection systems are so-called K-systems wherein a continuously injecting valve is precontrolled mechanically with respect to its base load by an air quantity meter and is corrected by means of a special positioning current originating at the lambda control.
On the other hand, the tank venting in the operationally warm condition may not be closed for a longer period of time. Usually, this leads to a known time control wherein adaptation takes place for blocked tank venting in alternation with the inhibition of learning in the presence of tank venting.
In a practical realization, it has been shown that the disturbing influence caused by the tank venting can be so large that it brings the active lambda control out of its control region for the two operating conditions of open and closed tank venting. That is, it permits operation at its one stop (rich) and this perhaps over a very long time span. Such a performance then makes necessary the introduction of one or more corrective values which return the control loop to .lambda.=1 and is therefore complex.
These considerations led to the solution disclosed in U.S. Pat. No. 4,683,861 of comparatively complex adaptive precontrol corrections for which the disturbing quantities were detected only below the load region by means of mean-value formation of the lambda controller and it was attempted to hold the percentage error constant with a precontrol characteristic field for the opening cross section of a tank venting valve. The learning value is weakened by means of a factor above this load threshold. The learn value has two stops at which the opening cross section of the tank venting valve or the time control for the performance of the base adaptation/tank venting is changed when these stops are reached. If no learning region is active, then the learned value is again unlearned over a certain time with a so-called forgetting factor. In addition, control conditions exist which act upon the control at several locations and include several time constants.